As an image forming apparatus such as a color laser printer, for example, has applied a method in which a plurality of image forming units are disposed side by side along a belt for conveying sheets, and toner images are transferred in sequence from each image forming unit onto a sheet that is conveyed on the belt. In this kind of image forming apparatus, a technique known as “registration” is employed to prevent the occurrence of a shift (color shift) in the transfer position between each image forming unit.
According to this technique, for example, a predetermined mark is formed on the belt by each image forming unit, a light is radiated onto the belt, and a reflection light therefrom is received by a sensor. The position (existence/non-existence) of a mark is then detected by reading a difference in the reflectance ratios between the belt surface and a mark portion based on the output of the sensor, and correction of a color shift is performed on the basis of that result.
FIG. 13 is a graph illustrating one example of the output of a sensor as described above. In this example, the sensor output increases at a portion M that corresponds to a mark on the belt because the reflectance ratio is low. At a portion B that corresponds to a portion of the belt surface on which a mark is not formed, the sensor output decreases because the reflectance ratio increases. The existence or non-existence of a mark on the belt is determined by comparing the sensor output with a predetermined threshold T1.
However, the surface of a belt sometimes becomes damaged or dirtied accompanying usage. In that case, the reflectance ratio of the belt surface decreases because the light reflects diffusely due to the damage or dirt. As a result, as shown in FIG. 14, there is a risk of the sensor output B from the belt surface increasing to a level that exceeds the threshold T1 such that it will no longer be possible to detect the marks normally.
The conventional technology attempts to solve the above described problem by, for example, changing the threshold to a threshold T2 as shown in FIG. 15. However, in this case, since the difference between the threshold T2 and the sensor output B from the belt surface and the difference between the threshold T2 and the sensor output M from the mark portion respectively decrease, there has been a problem that erroneous detection due to the influence of noise caused by damage or the like on the belt surface is liable to occur.
Because of the above described circumstances, there is a need for technology that can ensure detection accuracy when detecting marks are formed on a bearing member.